1. Field of the Invention
The present invention relates to a shadow mask in a color cathode ray tube, and more particularly, to a shadow mask in the color cathode ray tube, having a curvature structure with a reinforced center portion for implementing a flat image.
2. Background of the Related Art
In general, the cathode ray tubes are mainly used in image displays, such as TV receivers, or computer monitors. FIG. 1 illustrates a side view of a related art color cathode ray tube with a partial cut away view.
Referring to FIG. 1, the related art color cathode ray tube is provided with a fluorescent film 2 inside of a panel 1 having red, green and blue fluorescent material coated thereon, a funnel 3 welded to rear of the panel 1, and an electron gun 4 provided in a neck portion 3a of the funnel. There is the shadow mask 6 fixed to a frame 7 close to the fluorescent film 2 for selecting colors form the electron beams 5 emitted from the electron gun 4. The frame 7 is fixed to a sidewall of the panel 1 hung therefrom by means of a holding spring 8 fixed to the frame 7 and inserted in a stud pin 9 fixed to the sidewall. There is an inner shield 10 on one side on one side of the frame 7 fixed by a holding spring 1 for protecting the electron beams 5 moving toward the fluorescent film 2 from an external geomagnetic field. There are 2, 4, or 6 pole magnet 13 on an outer circumference of the neck portion 3a for adjusting a path of travel of the electron beams so that the electron beams 5 can hit onto a desired fluorescent material exactly, and there is a reinforcement band 12 wounded around on an outer circumference of the cathode ray tube for preventing damage from an external impact when the cathode ray tube is in operation. In the foregoing basic structure of the cathode ray tube, the shadow mask 6 is formed to have a designed radius of curvature and assembled with the panel with a fixed space between the panel 1 and the shadow mask 6, to form a panel assembly. The shadow mask 6 facilitates the three electron beams emitted from the electron gun 4 to hit on the fluorescent material inside of the panel 1 through the shadow mask 6 exactly, for reproducing an image.
FIG. 2 illustrates a cross section of a panel assembly, referring to which the curvature and layout of the shadow mask 6 will be explained in detail.
In general, the radius of curvature of the shadow mask 6 is designed such that a relation of 0.70 less than Rm/Rp is established with reference to a diagonal axis where Rp is an inside surface radius of curvature of the panel 1, and Rm is a radius of curvature of the shadow mask 6. Accordingly, provided that the inside surface radius of curvature Rp of the panel 1 is given, the radius of curvature Rm of the shadow mask 6 is primarily dependent on the inside surface radius of curvature Rp of the panel 1. Along with the primary dependence on the inside surface radius of curvature of the panel, the shadow mask 6 is designed taking a G/R (grouping rate) which fixes a color purity of the image into consideration, which G/R, an array of the electron beams (distance) can be expressed with the following equation with reference to FIGS. 3Axcx9c3C.       Beam    ⁢          xe2x80x83        ⁢    Grouping    ⁢          xe2x80x83        ⁢    Rate    ⁢          xe2x80x83        ⁢          (              G        /        R            )        =                    3        xc3x97        S        xc3x97        Q                    Ph        xc3x97        L              =                  B        A            xc3x97      1.5      
Where, S: a deflection yoke component
Q: a distance from a slot in the shadow mask to an inside surface of the panel
Ph: a center distance between slots in shadow mask(or a pitch of the slot=A)
L: a distance from an electron beam deflection center of a deflection yoke to an inside surface of the panel
B: a center distance to each of centers of R(red) and B(blue) beams from the center of G(green) beam
A: a pitch of the slot(Ph)
The beam grouping rate(G/R) in general color cathode ray tube is set such that the electron beams hit a desired fluorescent dot exactly all over the effective surface of the shadow mask to improve a color purity, i.e., set to G/R=1.000(Just). That is, if the inside surface radius of curvature Rp of the panel 1 is fixed, the deflection yoke, i.e., a deflection yoke component xe2x80x98Sxe2x80x99 is fixed, and an electron beam incident angle for each position is set according to the fixed deflection yoke component xe2x80x98Sxe2x80x99. And, as illustrated in FIG. 3C showing a relative array of the three beams passing through the slits in the shadow mask 6, a critical horizontal pitch xe2x80x98Phxe2x80x99 represented as xe2x80x98Axe2x80x99 is fixed to meet a screen resolution requirement for a given video signal, and once the panel 1, the funnel 3 and the deflection yoke are fixed, a distance xe2x80x98Lxe2x80x99 from a deflection center of the deflection yoke to the inside surface of the panel 1 is fixed. Therefore, if changes in the curvature and the radius of curvature Rm of the shadow mask 6 is required, as xe2x80x98Lxe2x80x99 and xe2x80x98Sxe2x80x99 are constants, the slop pitch Ph should be changed. According to the equation, in order to make the radius of the curvature Rm of the shadow mask 6 smaller under a state the array of electron beam is fixed, the pitch Ph of the slot 6a should be designed greater, but, in order to make the radius of curvature Rm of the shadow mask 6 is greater, the pitch Ph of the slot 6a should be designed smaller. For example, as shown in FIG. 4a, when the radius of curvature of the shadow mask 6 is made smaller, the slot horizontal pitch Ph is set so that the slot horizontal pitch Ph has a relation of Ph0 less than Ph1 less than Ph2 - - - Phn starting from a center to a periphery of the shadow mask 6. As shown in FIG. 4B, if the horizontal slot pitch Ph is set greater, due to a limitation of effective surface of the shadow mask 6, the vertical pitch is changed appropriately to have a relation of Ph00 greater than Ph0n, Phn/20≈Phn/2n, and Phn0 greater than Phnn. In conclusion, as discussed, since the beam grouping rate (G/R) is influenced from the inside surface radius of curvature of the panel 1 basically, the curvature of the shadow mask 6 is varied under a condition of fixed G/R, i.e., G/R=1.00, taking color purity characteristics into consideration. Accordingly, the curvature and the radius of curvature Rm of a conventional shadow mask 6 is dependent on the inside surface curvature of the panel 1 under the condition of the basic design criteria(0.70 less than Rm/Rp) and a fixed G/R(=1.00).
Recently, as the inside surface curvature at a center of the panel 1 has become greater for implementing a flat image, as shown in FIG. 5, the inside radius of curvature Rp of the panel becomes smaller as it goes from the center of the inside surface of the panel 1 towards ends of long axis (X-axis), short axis (Y-axis) and diagonal axis (D-axis). That is, the center portion of the panel 1 becomes flatter.
long axis radius of curvature Rpx: Rpx1 greater than Rpx2 greater than Rpx3 greater than Rpx4 greater than  - - -
short axis radius of curvature Rpy: Rpy1 greater than Rpy2 greater than Rpy3 greater than Rpy4 greater than  - - -
diagonal axis radius of curvature Rpd: Rpd1 greater than Rpd2 greater than Rpd3 greater than Rpd4 greater than  - - -
Accordingly, as discussed, because the curvature and the radius of curvature Rm of the shadow mask 6 is dependent on an inside curvature structure of the panel, a shadow mask having a great radius of curvature at a center thereof is designed. In such a[s] shadow mask 6, for example, when the center radius of curvature is greater than 3,300 mm, the shadow mask 6 becomes weak, and is susceptible to deformation by an external physical force in handling the shadow mask 6 in a fabrication process, or deterioration of howling characteristics by an impact or a speaker sound during operation of the cathode ray tube.
Accordingly, the present invention is directed to a shadow mask in a color cathode ray tube that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a shadow mask in a color cathode ray tube, which provides an appropriate picture quality for a set resolution and has an increased overall strength by a bi-curvatured structure with a reinforced center portion.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the shadow mask in a color CRT includes a curvature structure formed according to a curvature function g1(x,y), the curvature function g1(x,y) including a peripheral curvature function f1(x,y) of the shadow mask having a predetermined curvature and a central curvature function f2(x,y) having an almost flat curvature, both the predetermined curvature and the almost flat curvature being formed according to a curvature function f3(x,y) corresponding to an inside surface curvature of a panel, resulting in a curvature which is stable in view of structure.
The region of the maximum radius of curvature is set within a range xc2xdLxe2x89xa6Fxe2x89xa6⅔L for preventing deterioration of purity characteristics, where xe2x80x98Fxe2x80x99 denotes the region of the maximum radius of curvature and xe2x80x98Lxe2x80x99 denotes a distance starting from a center of the shadow mask to an end of a useful surface in any direction.
The curvature function g1(x, y) is an arithmetic average of the curvature functions of f1(x, y), f2(x, y) and f3(x, y).
The shadow mask has a long axis(X-axis) having a radius of curvature similar to an inside surface curvature of the panel, short axis and diagonal axis each having a radius of curvature fixed according to the curvature function g1(x, y).
The curvature structure according to the curvature function g1(x, y) has a beam grouping rate G/R which is a group rate(G/R less than 1) in a central portion of the shadow mask, and a de-group rate(G/R greater than 1) in a peripheral portion of the shadow mask.
The group rate(xe2x88x92X) is set approx. two time greater than the de-group rate(+X), wherein the beam grouping rate in overall is set within a range of 0.94xcx9c1.030 by setting the de-group rate(+X) to be max. 0.03 and the group rate(+X) to be max. xe2x88x920.06, for maintaining an appropriate purity.
Accordingly, the shadow mask in a color CRT of the present invention can improve an overall strength while an appropriate picture quality is maintained, thereby preventing deformation by an external force and howling caused by impact or speaker sound during operation of the CRT.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.